Circuit monitoring device

ABSTRACT

The circuit monitoring device is disclosed. The device is for monitoring circuit resistance. At configurable thresholds digital flags are triggered, the device can be used as a Security/Building management system. The device uses open technology is fully scaleable and allows programmable logic controllers to be used as security management systems. Using a soft logic option a PC could take the place of the PLC.

FIELD OF THE INVENTION

[0001] The present invention relates generally to monitoring systemsand, in particular, concerns a device, method and system for monitoringthe status of a circuit. The device is especially useful in securitymanagement systems, fire systems and building management systems, and itwill therefore be convenient to describe the invention in relation tothose example applications. It should be understood however that theinvention is intended for broader application and use.

BACKGROUND

[0002] Security management systems are typically employed incorrectional facilities, such as prisons, as well as buildings intendedfor other purposes where restricted access is required. Some examples ofsuch systems include those sold under the names Pagasus, Card key andAccess. In general, these systems are proprietary, and components fromone system will not work with components from another system.Additionally, any modifications to the hardware or software mustgenerally be made by the originally manufacturer.

[0003] In a typical prior art security management system (SMS) a numberof field devices, perhaps several hundred or even thousands, are wiredback via various circuits to a centralised SMS control unit. Typicalfield devices include infra-red motion detectors, read switches on doorsand windows, glass breakage tapes on windows, smoke or heat detectorsand tamper switches. Each of these field devices includes a switchableelement which is triggered when an abnormal or specified conditionoccurs, for example a read switch detects that a door is opened, aninfra-red motion detector senses movement or a smoke detector sensessmoke in the air. The switchable element may be a normally open contact(ie., it closes when triggered) or it may be a normally closed contact(ie., it opens when triggered).

[0004] In general, a first resistive component is connected in serieswith the switchable element and a second resistive component, referredto herein as a field resistor, is connected in parallel with theswitchable element. The field resistor is typically connected across theterminal block of the field device at the time of installation. If morethan one field device is connected within a particular circuit, theswitchable element of each of those devices is connected in parallelwith the field resistor. In this configuration, the field resistor isusually connected across the switchable element of the last field deviceon a line extending from the SMS control unit.

[0005]FIG. 1 shows a typical example of a single line circuit connectedto a switchable element SW1 of a single field device. The circuitincludes a first resistive component R1 in series with the switchableelement SW1 and a second resistive component R2 (field resistor) inparallel with the switchable element SW1. Several field devices may beconnected to this circuit and, in that event, the switchable elements ofthose field devices would be connected in parallel with the fieldresistor R2. In practice, the field resistor R2 would be connected tothe field device farthest from the input terminals 1, 2 of the SMScontrol unit.

[0006] On considering the circuit shown in FIG. 1, it will beappreciated that the line resistance measurable at input terminals 1, 2of the SMS control unit will change when the switch SW1 closes. With theswitch SW1 in the open position the line resistance will be R1 plus R2.With the switch SW1 in the closed position the line resistance will beR1 alone. The SMS control unit determines the status of the switch SW1(opened or closed) by continuously measuring the circuit resistance ofthe line connected to its input terminals 1, 2.

[0007] Each manufacturer of SMS equipment specifies a particular valueof field resistor to be connected across the last field device in aline. Typical values may be 2 kΩ, 4.7 kΩor 10 kΩ. The resistance of thecable itself is in general insignificant in comparison to the values ofthe resistive components R1 and R2 involved in the circuit. In manyapplications, the series resistor R1 is the same value as the fieldresistor R2. In any particular installation, wherein all lines areconnected to a single SMS control unit, the field resistor R2 for eachline of the system in the same value.

[0008] The various field devices in a particular installation are oftensupplied by other manufacturers and those devices can generally be usedwith any SMS control unit. This is because the field devices merelycontain a switching element and the field resistor is connected duringinstallation of the system. In some cases however, the supplier of theSMS control unit may also supply field devices and, in those cases, thefield resistor may be hard wired within the device, rather than beingexternally wired across the terminal block at the time of installation.In that event, the field devices can only be used with the same brand ofSMS control unit.

[0009] These factors cause a few problems when the owner of an SMSsystem needs to upgrade or modify its system. Because each lineconnected to the system includes a field resistor of a particular value,the owner is forced to return to the original supplier of the SMS inorder to provide an upgrade. Alternatively, the system owner must rewireeach of the lines connected to the system and replace the field resistorwith a different value, as specified by the supplier of the new SMScontrol unit. Where the resistor is built into the field device itcannot be changed and the system owner is forced to also replace each ofthe devices if it wants to change to a different brand of SMS controlunit.

[0010] Typical SMS systems include an operator interface providing agraphical representation of the system being monitored and controlled.The software employed in the interface is proprietary and cannot bechanged by the user. Any modification to the operator interface thusneeds to be made by the original supplier and this makes the ownervulnerable to excessive ongoing maintenance costs by the supplier.

[0011] In an attempt to remove this dependency on the original supplier,the present inventor has in the past developed a universal replacementfor a proprietary SMS system using a standard programmable logiccontroller (PLC) and analog input cards. This provided a flexiblesolution which could be programmed to cater for a wide variety of fieldresistor values. Any PLC could be used to replace the proprietary systemwithout having to change the field resistors, thus saving considerableinstallation time. The programming of the PLC is more time-consuming,because all processing is done within the central processor of the PLCand this needs to be programmed using conventional ladder logic, butoverall installation time is reduced. The main problem with thisapproach in a commercial installation, however, is the high cost ofanalog input cards for commercially available PLCs. The cost of thesecards makes this form of PLC-based SMS prohibitively expensive for largeinstallations.

[0012] There therefore remains a need for a flexible system which canreproduce the function of a security management system, or similarsystems, or which can be used in conjunction with standard and commonlyavailable hardware and software to provide the necessary functionality.

SUMMARY OF THE INVENTION

[0013] The present invention accordingly provides a device formonitoring the status of a circuit based on a measurable parameter ofthe circuit, the device including:

[0014] measurement means for measuring the parameter of the circuit;

[0015] comparison means for comparing the measured parameter to at leastone threshold value and for assigning a status based on the result ofthe comparison; and

[0016] output means for presenting an indication of the assigned status.

[0017] This device may be used to measure the electrical resistance of acircuit and, based on that measurement, provide the functionality of atraditional security management system.

[0018] In one embodiment, the circuit is an electrical circuitcontaining at least one switchable element. This switchable element maybe incorporated within a field device of the type described above. Thecircuit includes a first resistive component in series with theswitchable element and a second resistive component in parallel with theswitchable element such that the status of the switchable element isreflected in the circuit resitance.

[0019] In one embodiment the threshold value is adjustable by a user. Inthis way, the device is able to cater for a wide variety of values ofthe first and second resistive components. This enables the device to beretrofitted to existing SMS systems, wherein the resistors may have beeninstalled many years earlier and may not be readily accessible forreplacement.

[0020] Preferably, the comparison means includes a plurality ofthreshold values for assigning a corresponding plurality of statusconditions. In one embodiment, the plurality of status conditionsincludes the following:

[0021] short circuit,

[0022] alarm 2,

[0023] normal,

[0024] alarm 1, and

[0025] open circuit.

[0026] The device preferably also includes communication means forcommunicating the status to a monitoring system. The communication meanspreferably employs an open communication standard such as the DeviceNet™open network standard developed by the Open DeviceNet Vendor AssociationInc. DeviceNet™ is a low cost communications link used to connectindustrial devices (such as limit switches, photo electric sensors,process sensors, panel displays and operator interfaces) to a networkand eliminate expensive hard wiring. The direct connectivity providesimproved communication between devices as well as important device-leveldiagnostics not easily accessible or available through hard wired I/Ointerfaces. DeviceNet™ is a simple, networking solution that reduces thecost and time to wire and install industrial automation devices, whileproviding interchangeability of “like” components from multiple vendors.A description of the DeviceNet™ standard can be found in the July 2000DeviceNet™ Product Catalogue by Open Vendor Association, Inc. ThisProduce Catalog is incorporated herein by cross-reference.

[0027] Another aspect of the present invention provides a securitymanagement system incorporating a circuit monitoring device of the typedescribed above. Such a system may utilise standard programmable logiccontroller hardware together with standard operator interface softwareto provide a fully functional security management system. The circuitmonitoring device may be in the form of a separate module which isconnected to the PLC using a communications module based on theDeviceNet™ standard, or other suitable open communication standard.Alternatively, the circuit monitoring device may be configured as aplug-in card which connects directly to the back plane of the PLC. Inthis form, different versions of the circuit monitoring device wouldneed to be made to plug in to different brands of PLC. A separateDeviceNet™ module thus has the advantage that it can be used with anybrand of PLC.

[0028] A major advantage of the present invention is that it allows theretrofit of existing security management systems, fire systems andbuilding management systems, while utilising the existing circuit wiringregardless of existing resistance values. Retrofits and newinstallations may use various PLCs and operator interfaces, and avariety of hardware and software, instead of being locked intoproprietary hardware and software.

[0029] As a further alternative, the circuit monitoring device may bebuilt into a card which is adapted to plug directly into a personalcomputer or similar device.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Preferred embodiments of the present invention will now bedescribed with reference to the accompanying drawings. In the drawings:

[0031]FIG. 1 shows a circuit in a prior art security management system;

[0032]FIG. 2 shows a monitoring system-incorporating three embodimentsof the circuit monitoring device of the present invention;

[0033]FIG. 3 shows a circuit block diagram for one input of the circuitmonitoring device of the present invention;

[0034]FIG. 4 shows a diagrammatic representation of comparisons made todetermine status conditions according to the present invention;

[0035]FIG. 5 shows a circuit diagram for an end of line resistancemodule.

[0036]FIG. 6 shows a circuit diagram for a closed loop module; and

[0037]FIG. 7 shows a circuit diagram for a prototype circuit monitoringdevice in accordance with one embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0038]FIG. 2 of the drawings shows an example application of the circuitmonitoring device of the present invention. In this application a numberof circuit monitoring devices are used in a security management system(SMS) to monitor the status of various circuits containing field devicessuch as motion detectors, read switches on doors and windows, smokedetectors, etc. In particular, a centralised SMS control unit 5communicates with three monitoring devices 10, 20 and 30 to monitorthree individual electrical circuits labelled generally as A, B and C inFIG. 2 respectively.

[0039] The SMS control unit 5 includes a conventional programmable logiccontroller (PLC) such as an Allen Bradley model SLC 505 produced byRockwell Automation, or any other suitable model produced by anothermanufacturer such as Siemens, Omron, Mitsubishi, etc. The PLC includes amicroprocessor card 6 and may include various input and output cards orcommunications cards.

[0040] Circuit A includes a switchable element SWA associated with afield device (eg. an infra-red motion detector), a first resistivecomponent R1 in series with the switchable element SWA and a secondresistive component R2 in parallel with the switchable element SWA. Thesecond resistive component R2 is typically wired across the terminalblock of the field device at the time of installation and is oftenreferred to as a field resistor.

[0041] In this application, the circuit monitoring device 10 may becalled an “end-of-line resistance module (EOL module) because itmeasures the end-of-line resistance of circuit A. It is thus convenientto hereinafter refer to the device 10 in this way.

[0042] Similar to the conventional circuit shown in FIG. 1, theend-of-line resistance of circuit A will change when the switchableelement SWA closes or opens. The measured resistance may thus be used byEOL module 10 to determine whether the switch SWA is open or closed.Further, the EOL module 10 can determine the existence of a faultcondition such as an open circuit (infinite resistance) or short circuit(very low resistance).

[0043] The EOL module 10 is configured electrically and mechanically tobe plugged directly into the back plane of the PLC. This module may thusbe produced as a form of plug-in card, similar to conventional digitaland analog input and output cards. Communication between themicroprocessor 6 of the PLC and the EOL module 10 is via the back planeof the PLC.

[0044]FIG. 2 also shows two remote EOL modules 20 and 30. A scannermodule, being a communications card, is provided to enable communicationwith remote EOL modules 20 and 30.

[0045] EOL module 20 monitors the resistance of circuit B whilst EOLmodule 30 monitors the resistance of circuit C. Circuit B is identicalto circuit A but the EOL module 20 is remote from the PLC. EOL module 20employs the DeviceNet™ standard to communicate with the PLC via acommunications link 8 and DeviceNet communications card 7 which isplugged into the back plane of the PLC.

[0046] EOL module 30 is a closed loop form of resistance module whichmeasures the resistance of circuit C via inputs 1 and 2 and inputs 3 and4. This circuit provides an extra level of security in the event that asection of the circuit fails due to an open or short circuit. The EOLmodule 30 also operates according to the DeviceNet™ standard andcommunicates with the communications card 7 of the PLC viacommunications links 8 and 9.

[0047]FIG. 3 shows an example input circuit as may be used within anyone of the EOL modules 10, 20 or 30. The input circuit includes anoperational amplifier (OPAMP) 40, an analog to digital converter 41 (A/Dconverter), a microprocessor 42 and a communication module 43. A fieldcircuit, for example circuit A, B or C of FIG. 2, is connected to theinput of the OPAMP 40. An analog output of the OPAMP 40 is converted bythe A/D converter 41 to a count value representing its analog input.This count value is then a numerical representation of the end-of-lineresistance of the field circuit. The microprocessor 42 compares thevalue of the measured resistance with various thresholds to determinethe status of the field circuit, and of any switchable element withinthe field circuit. The result of this comparison is communicated to acentralised monitoring system such as the SMS control unit 5 shown inFIG. 2.

[0048] In the EOL module 10 (FIG. 2) the communication module 43 isadapted for communication across the back plane of the PLC to themicroprocessor 6. In EOL modules 20 and 30 (FIG. 2) the communicationmodule 43 is a DeviceNet™ communication module implementing theDeviceNet™ communication standard.

[0049] For the sake of simplicity, FIG. 3 shows a single field circuitconnected to a single A/D converter, microprocessor and communicationsmodule. However, in practice, an EOL module would include multipleinputs, for example, 8 or 16. In the case of a 16 input EOL module,sixteen OPAMP may be used and these may be connected respectively to 16A/D converters. However, the outputs from the sixteen OPAMPS mayalternatively be multiplexed to a single A/D converter. A singlemicroprocessor may be used to read each of the digital resistance valuesto determine a status condition for each of the field circuits.

[0050]FIG. 7 shows a circuit diagram for a prototype circuit monitoringdevice. The device provides for eight input circuits connected to aneight channel analog to digital converter. This is connected via an I/Obus to a central processing unit (CPU) which is in turn connected to aDeviceNet™ communication module.

[0051]FIG. 4 shows a diagrammatic representation of the comparisons madeby the microprocessor 42 (FIG. 3) for a field circuit. This exampleassumes that the EOL module uses a 16 bit A/D converter. Such aconverter produces a count value ranging from 0 to 32,767. This countrepresents the measured end-of-line resistance of the field circuit. Thecount is compared to various thresholds, as shown, to determine a statuscondition for the field circuit. If the count is below 8,000, an OpenCircuit condition is assigned. If the count is above 30,000, a ShortCircuit condition is assigned. A value between 15,000 and 16,000 isconsidered to be the normal operational range for the circuit, and aNormal condition is assigned. Values between 8,000 and 15,000 areassigned an Alarm 1 condition whilst values between 16,000 and 30,000are assigned a Alarm 2 condition.

[0052] Referring now to circuit A in FIG. 2, and assuming that switchSWA is a normally open switch, one would expect the normal end-of-lineresistance of the circuit to be equal to the values of R1 plus R2. Thisresistance value would produce a count between 15,000 and 16,000 in FIG.4. A range of count values are specified in order to allow forvariations in the circuit resistance resulting from cable resistance andconnections. Some variation would clearly occur depending on the lengthof the cable extending to the field devices and the cross-sectional areaof those cables. When the switch SWA closes, the end-of-line resistancewould drop to the value of R1 alone. In FIG. 4, this would produce aAlarm 2 condition. Alternatively, if the switch SWA was instead anormally closed, that condition would be considered “normal” and openingthe switch SWA would result in an increase in the end-of-line resistanceto the value of R1 plus R2. This would produce an Alarm 1 condition inFIG. 4. Thus, what is considered “normal” depends on the type ofswitchable element used in the field circuit. It will also beappreciated that the definition of High and Low in FIG. 4 could bereversed compared to the scenario just described.

[0053] The EOL module 10 can also detect the presence of a faultcondition, such as an open circuit or a short circuit. In the case of ashort circuit, the end-of-line resistance drops to a very low value,depending upon the resistance of the cable and the location along thecable of the short circuit. In the case of an open circuit, theresistance increases to a very high value, dependent upon the resistanceof the insulation of the cable. A range of values is thus used to allowfor such variations.

[0054] It is considered that appropriate software for the microprocessor42 shown in FIG. 3 may be written by any skilled computer programmerand, accordingly, need not be described herein in detail. The languageused may be a high level language or a low level machine languageappropriate to the particular microprocessor used in the EOL module.

[0055] The various threshold values shown in FIG. 4 at 8,000, 15,000,16,000 and 30,000 are preferably configured as variables which may beset as parameters of the EOL module. In this way, the EOL module may beconfigured to operate with a wide range of field resistors, thusenabling the EOL module to be retrofitted to a wide range of fieldcircuits wherein the series and field resistors (R1, R2 respectively)already exist and cannot readily be changed.

[0056] After comparing the measured resistance to each of the thresholdvalues the microprocessor 41 (FIG. 3) produces, as an output, anindication of the status of the field circuit, eg. circuit A, B or C inFIG. 2. This output may be in the form of individual flags or bits whichare set when a particular status condition is assigned and thus has onlytwo possible values from each comparison. For example, five output bitsmay represent five possible status conditions, namely Short Circuit,Alarm 2, Normal, Alarm 1 and Open Circuit.

[0057] Thus, in accordance with an embodiment of the invention, the EOLmodule measures the end-of-line resistance of the field circuit,compares the measured resistance to a number of threshold values andassigns a status based on the result of the comparison. This status isthen presented as an output in the form of five digital bits which thencan be read by or transmitted to a centralised monitoring system. Thiscentralised system does not need to concern itself with the actual valueof the end-of-line resistance for the circuit but merely with thedetermined status of the circuit. This is significant because merely afew bits of information needs to be transferred, rather than a wholeword representing the analog value. In FIG. 2, the microprocessor 6 ofthe PLC merely needs to read 5 flags or bits from EOL module 20, via thecommunications module 7. The microprocessor 6 is not concerned with, andis not even aware of, the actual end-of-line resistance of the circuit Bwhich is connected to the EOL module 20. The communications module 7,being a conventional scanner module produced by the manufacturer of thePLC equipment, scans the EOL module 20 using conventional DeviceNet™standards.

[0058] To configure a particular EOL module, such as a module 20 in FIG.2, the threshold values are controlled by software at the module level.For example, using software called RS Networks (Rockwell SoftwareNetworks) produced by Rockwell Automation, it is possible to access anyparticular module connected to the PLC network. The RS Networks softwaredisplays the parameters of each of those modules and the parameters canthen be changed. In the present application, the threshold values (shownin FIG. 4) may be changed as parameters of the DeviceNet™ EOL module 20.Once the parameters are set, they are stored within the module 20, notthe PLC, and are retained within non-volatile memory of that module.

[0059] In one form, the parameters may be set individually for eachinput of a multi-input module. However, more likely, the parameterswould be identical for each input of the module and each, at leastinitially, would be set using the same parameters. Individual changescould be made after setting the default parameter for the whole module.

[0060] The EOL modules may also be programmed with default thresholdvalues at the time of manufacture. For example, the threshold value maybe set at levels appropriate for field circuits employing fieldresistors having a value of 4.7 kΩ. In this way, the EOL module may beused in a PLC-based retrofit, for a conventional security managementsystem which normally uses field resistors having a value of 4.7 kΩ,without needing to program the EOL modules at all. If the system beingreplaced uses field resistors having a different value, then the EOLmodules can be reprogrammed for that value.

[0061]FIGS. 5 and 6 show extended versions of circuits B and C in FIG. 2respectively. In each of FIGS. 5 and 6 a number of field devices areconnected within the circuit. Like reference numerals are used in FIGS.5 and 6 to represent like component in FIG. 2. The field devices may besmoke detectors, read switches or other forms of detector.

[0062] A PLC based security management system would preferably beprovided with an operator interface in the form of a visual display unitand an input device, such as a computer keyboard. A visualrepresentation of the system being monitored would be presented on thevisual display. A number of standard Supervisory Control And DataAcquisition (SCADA) software packages are available which can be run onstandard personal computer (PC) hardware. Some examples include FIX byintellution, Citec by CI Technologies. Alternatively, a customised userinterface may be developed using graphical programing tools such asActive X, Visual Basic or Visual C++. The personal computer may benetworked to one or more PLCs to provide an integrated securitymanagement system.

[0063] Similar PC and PLC hardware and software may be employed tocreate a fully functional fire system or building management system.

[0064] Such PC/PLC-based systems using EOL modules according to thepresent invention may be readily retrofitted to existing systems, whileutilizing the existing circuit wiring regardless of existing resistancevalues. A system built in this way, either as an original installationor as a retrofit, provides a flexible and relatively inexpensive optionwhich eliminates dependency on proprietary hardware and software.

[0065] A system employing the present invention provides various optionsincluding:

[0066] End-of-line resistance (as shown in FIG. 5);

[0067] Closed loop resistance (as shown in FIG. 6);

[0068] Dual redundancy,-end-of-line or closed loop (see below);

[0069] Intrinsically safe (see below).

[0070] Dual redundancy may be provided at various levels. For example,two communication lines may be provided between a communications scannermodule in the PLC and a remote EOL module. If one of the lines fails,the other keeps going. Alternatively, or in addition, two scannermodules may be provided in the PLC. Further, two microprocessors may beprovided within the PLC in critical application. Such dual redundantsystems are typically required in specialized fire systems.

[0071] Intrinsically safe systems are often required in hazardouslocations. This may be achieved by using an intrinsically safe barrieror module, which are commonly available, or by making the EOL moduleitself intrinsically safe. This saves on added wiring and additionalhardware costs but would make the cost of the module itself somewhatgreater.

[0072] Although preferred embodiments of the invention have beendescribed herein in detail, it will be understood by those skilled inthe art that variations may be made thereto without departing from thespirit of the invention or the scope of the amended claims. For example,the DeviceNet™ standard has been referred to herein for providing thecommunication link between a remote EOL module and a PLC communicationscanner module. There are, however, various communication networks whichmay be just as efficient. Such variations to the described system areconsidered to fall well within the scope of the appended claims.

1. A method of monitoring the status of a measurable parameter of anelectrical circuit, including the steps of: measuring the magnitude ofsaid parameter and generating an analog signal representative of saidmagnitude; passing said analog signal to an analog to digital converter;generating by the analog to digital converter a count valuerepresentative of said magnitude; comparing said count value with athreshold value and assigning from the comparison a status signal, saidstatus signal having two possible values which thereby indicates whetherthe count value is greater than or less than said threshold value;transmitting said status signal via a communications network to a isdisplay; and displaying on the display an indication of the value of thestatus signal.
 2. A method as defined in claim 1 wherein the thresholdvalue is adjustable by a user.
 3. A method as defined in claim 2 whereinthe threshold value is adjustable by means of configuration software. 4.A method as defined in any one of claims 1 to 3 wherein said comparisonis made with a plurality of threshold values and a status signal isassigned in respect of each of said threshold values, each of saidstatus signals are transmitted via a communications network to adisplay; and an indication of the value of each of the status signals isdisplayed on the display.
 5. A method as defined in claim 4 wherein saidplurality of status signals indicate respectively the following: shortcircuit, alarm 2, normal, alarm 1, open circuit.
 6. A method as definedin any one of the preceding claims, wherein said transmission of thestatus signal includes a wireless communication step.
 7. A method asdefined in claim 6 wherein the wireless communication employs theDeviceNet open network standard.
 8. Apparatus for monitoring the statusof a measurable parameter of an electrical circuit, the apparatuscomprising: measurement means for measuring the magnitude of saidparameter and generating an analog signal representative of saidmagnitude; analog to digital conversion means for generating from saidanalog signal a count value representative of said magnitude; comparisonmeans for comparing said count value with a threshold value andgenerating from the comparison a status signal, said status signalhaving two possible values which thereby indicates whether the countvalue is greater than or less than said threshold value; transmissionmeans for transmitting said status signal via a communications networkto a display; and display means for displaying an indication of saidassigned status.
 9. Apparatus as defined in claim 8 wherein the circuitis an electrical circuit containing at least one switchable element, thecircuit including a first resistive component in series with theswitchable element and second resistive component in parallel with theswitchable element such that the status of the switchable element isreflected in the circuit resistance, and wherein the measurableparameter is the circuit resistance.
 10. Apparatus as defined In claim 8or 9 wherein the threshold value is adjustable by a user.
 11. Apparatusas defined in claim 10 wherein the threshold value is adjustable bymeans of configuration software.
 12. Apparatus as defined in claim 10 or11 wherein the threshold value is held in non-volatile memory within theapparatus.
 13. Apparatus as defined in any one of claims 8 to 12 whereinthe comparison means includes a plurality of threshold values forassigning a corresponding plurality of status signals.
 14. Apparatus asdefined in claim 13 wherein said plurality of status signals indicaterespectively the following: short circuit, alarm 2, normal, alarm 1,open circuit.
 15. Apparatus as defined in any one of claims 8 to 14wherein said transmission means includes a wireless communication stage.16. Apparatus as defined in claim 15 wherein the transmission meansemploys the DeviceNet open network standard.
 17. A security managementor building management system including apparatus as defined in any oneof claims 8 to
 16. 18. A fire system including apparatus as defined inany one of claims 8 to
 16. 19. A building security management systemcomprising: an electrical circuit containing a switchable element, thecircuit including a first resistive component in series with theswitchable element and a second resistive component in parallel with theswitchable element such that the status of the switchable element isreflected in the electrical resistance of the circuit; measurement meansfor measuring said resistance of the circuit and generating an analogsignal representative of said resistance; comparison means for comparingsaid count value with a threshold value and generating from thecomparison a status signal, said status signal having two possiblevalues which thereby indicates whether the count value is greater thanor less than said threshold value; transmission means for transmittingsaid status signal via a communications network to a display; anddisplay means for displaying an indication of said assigned status andthus of said switchable element.
 20. A security management system asdefined in claim 19 wherein the threshold value is adjustable by a user.21. A security management system as defined in claim 20 wherein thethreshold value is adjustable by means of configuration software.
 22. Asecurity management system as defined in claim 20 or claim 21 whereinthe threshold value is held in non-volatile memory within the comparisonmeans.
 23. A security management system as defined In any one of claims19 to 22 wherein the comparison means includes a plurality of thresholdvalues for assigning a corresponding plurality of status signals.
 24. Asecurity management system as defined in claim 23 wherein said pluralityof status signals indicate respectively the following: short circuit,alarm 2, normal, alarm 1, open circuit.
 25. A security management systemas defined in any one of claims 19 to 24, wherein said transmissionmeans includes a wireless communication stage.
 26. A security managementsystem as defined in claim 25 wherein said transmission means employsthe DeviceNet open network standard.
 27. A fire alarm system including asecurity management system as defined in any one of claims 19 to 26.